The increasing number of radio services will increase the need for more advanced solutions when it comes to an effective usage of antenna system infrastructure. The most important reason behind is probably economical but there are also other reasons like less visual impact and that many attractive antenna sites are already in use for other/old services.
New radio systems are continuously being developed. When a new system is introduced there is often a must to keep existing system for quite a long period. Existing systems have often attractive sites from both a propagation and capacity perspective making it very cost efficient to reuse as much as possible off existing site infrastructure.
There is also very often a wish to increase capacity off existing sites by adding/using more frequency spectrum. This makes it possible to delay the introduction of more sites and tighter site to site distances as long as extra capacity can be added without too much radio performance degradation. System improvement like advanced frequency hopping or new modulation methods like in WCDMA will also allow more spectrum on each site.
The simplest way and today most common way to solve above problems is to add more and more antennas on existing sites. New antenna technology like having more antennas, dual and even triple band antennas in the same housing has made this solution quite attractive. This solution even if being common is not always possible, typically it will add weight and wind load to the towers not only because of the more and more complex antennas but also because of the need for extra feeders. Feeder sharing is today only easy if different radio frequency bands with large guard-bands are combined. (Like Low band/High band) Adding antennas and feeders will of course also add costs.
If frequency bands or slots with a narrow guard band have to bee combined this can be done in hybrids or similar but these types of solution will introduce high losses making the solution less interesting for sites having a need for both high capacity and high coverage.
There is also a possibility to use what is called filter combiners making it possible to combine specific carriers or frequency bands to a common feeder. This technique has been popular in the past for systems using rather high number off carriers per site/sector. This technique is not that useful if having advanced frequency hopping or frequency bands divided in many small frequency slots having different bandwidths.
Existing band filter combiners are not flexible since they are often hard to retune in frequency and in most case impossible to change in bandwidth. For many operators this means that they have to have a high number off different versions and if there are changes in the spectrum allocation they often have to do replacements or expensive frequency retuning.
Low power combining plus high power amplifiers is another type of solution on the same problem. The complexity of these amplifiers makes a passive filter solution in many cases more attractive if comparing reliability and efficiency. The proposed solution can of course be used together with high power amplifiers. The solution will make it possible to combine these types of amplifiers in an effective way.
Most antenna systems today are using full duplex on the feeders. If using traditional band filter combiners it is necessary to bypass the up-link if the combiner is external to the radio base station.
One very good example showing the complexity to handle different band segments is the PCS 1900 MHz band in North America. It is a lot off band segments, the operators are buying extra slots, and the operators are buying each other sometimes forcing them to sell out a part of the band. A filter combiner not being flexible is quite a bad solution for this scenario.
EP1217733A1 presents a solution for a combination of a narrowband signal with a broadband signal to a common output. The narrowband branch has a passband filter. The design results in the narrowband signal meeting a parallel circuit of the antenna and the broadband branch. In order to not loose effect of the narrowband signal to the broadband branch, a stopband filter is provided on the broadband branch. A disadvantage with the solution in EP1217733A1 is that the lengths of the transmission lines between the respective filters and the interconnection between the narrowband branch and the broadband branch are critical. This makes it very hard to make a broadband solution with high return loss values over wide frequency ranges. The solution in EP1217733A1 is optimised to combine one or many narrow band signals to one more broadband port. This solution is not appropriate to use together with advanced frequency hopping for systems using the narrow band ports since it will give an unrealistic number of different bandpass filter for all used hopping frequencies.
When combining broadband ports with a high number of cavities per filter the tuning becomes very critical because of the critical filter matching via these transmission lines. This is most important for combiners optimised for easy or even remote retuning without having access to advanced instruments like network analysers and skilled personal.
In other words, changing the center frequency of the broadband branch in the solution according to EP1217733A1 will necessitate changing the length of these transmission lines, with in practice is not feasible.
FIG. 1 shows a sketch depicting a solution for broadcast applications. The sketch in FIG. 1 is based on a picture that was retrieved by one of the inventors on Feb. 9, 2005, from the internet-address www.dielectric.com/broadcast/cim.htm. This address was included as a link in a section with the heading “FM Combiners” in the address www.dielectric.com/broadcast/combiner.asp. The latter address mentioned the company Dielectric Communications, in Raymond, Me., USA. The sketch in FIG. 1 contains exactly the same content as the original picture and has been adapted only to meet the formal requirements of this application.
Thus, FIG. 1 shows a so called Constant Impedance Module of a FM Combiner. It can be seen that the module comprises two transmission lines, depicted as horizontal and mutually parallel in FIG. 1. The transmission lines are interconnected by two 3 dB hybrids. Between the hybrids, distributed on each transmission line, FIG. 1 depicts a set of square blocks, which appear to the skilled person, directly and unambiguously, using common general knowledge, as filters.
Referring to FIG. 1, at the right end of the upper transmission line a wideband signal is received, and at the left end of the upper transmission line a signal called F1 is received.
FIGS. 2 and 2a show schematic sketches for depicting the manner in which the module in FIG. 1 works. Referring to FIG. 2, by means of the left hybrid 1, F1 is divided into two signals F1a and F1b, each with half the effect of F1. F1a propagates in the upper transmission line 2 and F1b is sent via the left hybrid 1 to the lower transmission line 3.
When F1a reaches the right hybrid 4, it is divided into two signals F1aa and F1ab, each with half the effect of F1a. F1aa continues towards the wideband input 5 and F1ab is sent via the right hybrid 4 to wideband output 6.
When F1b reaches the right hybrid 4, it is divided into two signals F1ba and F1bb, each with half the effect of F1b. F1bb is sent via the right hybrid 4 towards the wideband input 5 and F1ab continues to wideband output 6.
The skilled person realises, directly and unambiguously, using common general knowledge, that the left hybrid 1 is adapted so that F1b is phase shifted 90° in relation to F1a, and that right hybrid 4 is adapted so that F1bb is phase shifted 90° in relation to F1ba, and therefore, F1bb is phase shifted 180° in relation to F1aa. Since F1bb and F1aa has the same effect, (¼ of the F1 effect), and are phase shifted 180° in relation to each other, they will be eliminated. Thereby, no effect from the F1 signal is sent to the wideband signal source.
Further, F1ab is phase shifted 90° in relation to F1aa, i.e. obtains the same phase as F1ba. Thereby, the signals of F1ab and F1ba will be added when sent to the wideband output 6.
FIG. 2a shows schematically the right part of the module in FIG. 1. By means of the right hybrid 4, a wideband signal W received at the wideband input 5 is divided into two signals Wa and Wb, each with half the effect of W.
Wa propagates in the upper transmission line 2, where a first filter 7 is located. The first filter 7 reflects the signal Wa, the reflected signal being referred to as WaR in FIG. 2a. Wb propagates in the lower transmission line 3, where a second filter 8 is located. The second filter 8 reflects the signal Wb, the reflected signal being referred to as WbR in FIG. 2a. 
When WaR reaches the right hybrid 4, it is divided into two signals WaRa and WaRb, each with half the effect of WaR. WaRa continues towards the wideband input 5 and WaRb is sent via to wideband output 6.
When WbR reaches the right hybrid 4, it is divided into two signals WbRa and WbRb, each with half the effect of F1b. WbRb is sent via the right hybrid 4 towards the wideband input 5 and WbRa continues to wideband output 6.
The right hybrid 4 is adapted so that Wb is phase shifted 90° in relation to Wa, so that WaRb is phase shifted 90° in relation to WaRa, and so that WbRb is phase shifted 90° in relation to WbRa. Therefore, WbRb is phase shifted 180° in relation to WaRa. Since WbRb and WaRa has the same effect, (¼ of the W effect), and are phase shifted 180° in relation to each other, they will be eliminated. Thereby, no effect from the W signal is sent back to the wideband signal source. Further, WaRb and WbRa are each phase shifted 90° in relation to W. Thereby, the signals WaRb and WbRa will add in phase when sent to the wideband output 6.
The skilled person realises, directly and unambiguously, using common general knowledge, that a number of hybrid alternatives can be used for the module shown in FIG. 1, including Magic T:s, described for example in “Kompendium i Mikrovgsteknik, Allmän kurs” Institutionen för Mikrovgsteknik KTH, (Royal Institute of Technology, Stockholm, Sweden), which was student literature at KTH in 1975.
The module shown in FIG. 1 is adapted for broadcast transmissions. However, in mobile communications, this solution does not fulfil practical requirements. One reason is that in practice there will always be a mismatch between the combiner and the antenna, which will cause signal reflection from the antenna to the wideband input. More particularly, antennas do not have a constant impedance over used frequency bands. It is possible to fine-tune the impedance for a narrow frequency band but for a wide bandwidth this is not realistic and as a result there will be a mismatch between the characteristic impedance of the feeder system and the antenna. As a result a portion of the energy is reflected back from the antenna. In practice there is also often provided boxes like diplexers or tower-mounted amplifiers in between the combiner and the antenna. These extra boxes will in most cases make the impedance matching even harder.
Another disadvantage with the solution shown in FIG. 1 is that it does not meet the above described need for flexibility regarding frequency retuning, combined with the need for high power levels of the signal (F1 in FIG. 1) transmitted through both hybrids. In particular, the hybrids of the module in FIG. 1 are adapted to provide the desired phase shifts at a certain frequency of the signal transmitted through both of them. If this frequency is changed, the above described phase shifts become not optimal for obtaining the desired elimination of the signal from the combiner to the wideband input. Thus, the solution will not provide full isolation of the wideband input.
The additional need in mobile communication applications for high effect of the signals will add to this problem. In mobile communication applications, it is a requirement that any unwanted signal leakage from one input port to another input port must low enough over the whole used frequency band. This requirement is also important to fulfil together with rather bad return loss for the components connected to the different ports. Signal leakage is critical because it will typically result in unwanted frequency generation when two or more frequency bands are mixed because of unlinearities.